[Pw_forum] Pressure-induced phase transition

[Amin Torabi]

Dear Hongsheng,

I don't understand why I should use relax instead of vc-relax. As far as I
understand, when a system is under pressure, both the atomic positions
inside a unit cell, as well as the dimensions of the unit cell changes. So,
why should I keep the unit cell parameters fixed if I am interested in
finding phase transitions?

Thanks

On Mon, Jul 18, 2011 at 4:04 AM, Hongsheng Zhao <zhaohscas at yahoo.com.cn>wrote:

> On 07/18/2011 09:40 AM, Amin Torabi wrote:
> > Dear experts!
> >
> > Forgive me for my naive question... but I am interested in
> > pressure-induced phase transitions of diborane. Could you please have a
> > look at my input file, attached to this message.
> >
> > The following is what I think I should do: (correct me if I'm wrong!)
> >
> > 1. doing separate vc-relax calculation at each pressure
>
> By using this method, there are some limitations.  You should have known
> in advance the different phases between them the phase transition maybe
> occur when we change the pressure.  Then you can use this method to
> determine the phase transition path.  But I'm not so sure this method
> can predict all of the possible phases for a system when we change the
> external pressure.  To do a more reasonable and complete phase
> transition prediction and even searching for unknown phases, you should
> use some third-party code like uspex for this type of job.
>
> In addition, I think you should do relax, not vc-relax calculation at
> each pressure.  If you do vc-relax, then the pressure imposed to the
> system will be released.  So you won't get the *Pressure-induced* phase
> transition.
>
> > 2. comparing the optimized cell_parameters at each pressure to see when
> > the structure changes
> >
> > I am wondering how I can use the enthalpy or unit cell volume at each
> > pressure to make sure that a phase transition has occurred
>
> For the other questions, see this thread on this list:  About the method
> to obtain the common tangent for determing the phase transition
> pressure.  HTH.
>
> Regards
> >
> > Thanks in advance
> >
> >
> > --
> > Amin Torabi
> > Ph.D. Student
> > Chemistry Department
> > /The/ University /of/ Western Ontario
> > London, On Canada, N6A 5B7
> > Phone: 519-661-2111 Ext: 87871
> > ****************************************
> >
> >
> >
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>
>
> --
> Hongsheng Zhao <zhaohscas at yahoo.com.cn>
> School of Physics and Electrical Information Science,
> Ningxia University, Yinchuan 750021, China
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>



-- 
Amin Torabi
Ph.D. Student
Chemistry Department
*The* University *of* Western Ontario
London, On Canada, N6A 5B7
Phone: 519-661-2111 Ext: 87871
****************************************
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